1. Field
The present invention relates to an optical element used for planar light sources and an illumination device using the optical element.
2. Description of the Related Art
Illumination devices using planar light sources such as light emitting diodes (LED) are widely utilized. Backlight units with LEDs for displays are required to illuminate the display uniformly. Compared with incandescence lamp light sources, in LED light sources a greater amount of light to the total amount of light emitted by the light sources travels to the front. Accordingly, in order to illuminate a large area uniformly with LED light sources, the number of the LED light sources should be increased in such a way that light beams emitted by the LED light sources largely overlap one another, or the illuminated area should be located away from the LED light sources. However, if the number of the LED light sources is increased, the manufacturing cost as well as power consumption will increase. If the illuminated area is located away from the LED light sources, the illumination device cannot be made compact in size. Under the situation, a backlight unit in which optical elements are arranged in front of LED light sources in such a way that a distribution of light emitted by each of the LED light sources (a distribution of luminous intensity) is appropriately changed has been developed (see, for example, JP3875247).
An LED light source for large quantity of light often includes a plurality of LED chips. In this case, the plurality of LED chips is often arranged in a line to suit the convenience of electrical wiring or other structural requirements. As a result, length of lengthwise direction and length of breadthwise direction of the LED light source are different from each other. Further, in some LED light sources which include a single LED chip, length of lengthwise direction and length of breadthwise direction of the LED light source are different from each other because of electrical wiring or other requirements.
However, an illumination device used for planar light sources which gives uniform illumination on an illuminated plane and in which length of lengthwise direction and length of breadthwise direction are different from each other, has not been developed.